A test point may be used to measure the performance of an upstream or downstream connection in a network. In one implementation, two separate directional couplers are used to connect two test points (TPs) to measure an upstream and a downstream signal. One directional coupler for a first test point is used for the downstream direction and a second directional coupler for a second test point is used for the upstream connection. This leaves an isolating port that is terminated by a termination load, such as a 75-ohm termination load if the test point also has a load of 75-ohms. The coupler directivity is limited by how good the isolating port is terminated and this achieves maximum directivity performance because the load of the isolating port is the same as the load of the test point.
In a full duplex node design, the use of two separate couplers may add too much of a loss at an output of a power amplifier connected to the coupler. With use of two separate couplers, the insertion loss will be doubled, from 1 dB in single coupler case to 2 dB in two coupler case. The output RF amplifier may already be running very close to its clipping point. With the use of a single coupler, the RF amplifier will not need to provide as much amplification compared to the two coupler case, which will improve the system level modulation error ration or bit error ratio (MER/BER) performance. Accordingly, a single bi-directional coupler may have to be used for both the upstream direction and the downstream direction. One advantage of using a bi-directional coupler is there is a lower insertion loss in the connection because only one coupler is used in both the upstream and the downstream directions. However, isolation between the upstream connection and the downstream connection may be an issue. The directivity of a coupler is defined as the power difference at any given coupling port when the same amount of power is injected into either the downstream direction or the upstream direction. A coupler may have around 25-30 decibels (dB) directivity, which means there may be 25-30 dB isolation between the upstream direction and the downstream direction at any given coupling port. However, this is based on an ideal 75-ohm termination at the isolating port. In a bi-directional coupler, the termination at the isolating port is typically not the perfect 75-ohm termination and thus will limit the isolation between the downstream and the upstream to the return loss of the isolating port. For example, the ideal isolation may not be achieved because one or both of the upstream test point and the downstream test point may not be connected to the bi-directional coupler. For example, a user may only be using one of the test points to test network traffic in one direction. This leaves an open test point port. In the full duplex node design, a 20 dB test point is required and a 10 dB coupler is used. The best return loss from the open test point port is 20 dB, which would limit the coupler directivity at 20 dB even when the coupler has 30 dB directivity by design when all ports are properly terminated. In the worst case, when two reflections are added in phase from two open test point ports, the total isolation between the downstream connection and the upstream connection could be as low as 16 dB.